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waves created by the wake of a ship. Asingle peak may
only require a mile or two in the dimension perpendicular
to the wind for high-amplitude lee waves to form,
though the wave lift will be confined to a relatively
small area in these cases.
LIFT DUE TO CONVERGENCE
Convergence lift is most easily imagined as easterly
and westerly winds meet. When the air advected by the
two opposing winds meet, it must go up. Air does not
need to meet “head on” to go up, however. Wherever
air piles up, it leads to convergence and rising air.
[Figure 9-30]
Examples of converging air leading to rising air have
already been discussed though not specifically referred
to as convergence. In Figure 9-17, convergence along
the outflow leads to air rising into the multi-cell thunderstorm.
In Figure 9-13, the circulation associated
with cloud streets leads to convergence under the
cumulus. A synoptic-scale example of convergence is
found along cold fronts. Convergence can occur along
distinct, narrow lines (convergence or shear lines), as
in Figure 9-30 (A), or can cause lifting over an area
several miles across (convergence zones), as in Figure
9-30 (B). At times convergence lines produce steady
lift along a line many miles long, while at other times
they simply act as a focus for better and more frequent
thermals.
One type of convergence line commonly found near
coastal areas is the so-called sea-breeze front. Inland
areas heat during the day, while the adjacent sea maintains
about the same temperature. Inland heating leads
to lower pressure, drawing in cooler sea air. As the
cooler air moves inland, it behaves like a miniature
shallow cold front, and lift forms along a convergence
line. Sometimes consistent lift can be found along the
sea-breeze front while at other times it acts as a trigger
for a line of thermals. If the inland air is quite unstable,
Wavelength
A Wavelength
B
Figure 9-29. Constructive and destructive interference.
Figure 9-30. Convergence examples. (A) Wind from different
directions. (B) Wind slows and “piles up.”
A
B
9-24
the sea-breeze front can act as a focus for a line of thunderstorms.
Additionally, since the air on the coast side
of the sea-breeze front is rather cool, passage of the
front can spell the end of thermal soaring for the day.
Sea air often has a higher dew point than drier inland
air. As shown in Figure 9-31, a “curtain” cloud sometimes
forms, marking the area of strongest lift. Due to
the mixing of different air along the sea-breeze front, at
times the lift can be quite turbulent. At other times,
weak and fairly smooth lift is found.
Several factors influence the sea-breeze front character
(e.g., turbulence, strength, and speed of inland penetration,
including the degree of inland heating and the
land/sea temperature difference). For instance, if the
land/sea temperature difference at sunrise is small and
overcast cirrus clouds prevent much heating; only a
weak sea-breeze front, if any, will form. Another factor
is the synoptic wind flow. A weak synoptic onshore
flow may cause quicker inland penetration of the seabreeze
front, while a strong onshore flow may prevent
the sea-breeze front from developing at all. On the
other hand, moderate offshore flow will generally prevent
any inland penetration of the sea-breeze front.
Other sources of convergence include thunderstorm
outflow boundaries already mentioned. Since this type
of convergence occurs in an overall unstable environment,
it can quickly lead to new thunderstorms. More
subtle convergence areas form the day after ordinary
thunderstorms have formed. If an area has recently
been subject to spotty heavy rains, wet areas will warm
more slowly than adjacent dry areas. The temperature
contrast can give rise to a local convergence line, which
acts similar to a sea-breeze front, and may be marked
by a line of cumulus.
Convergence can also occur along and around mountains
or ridges. In Figure 9-32(A), flow is deflected
around a ridgeline and meets as a convergence line on
the lee side of the ridge. The line may be marked by
cumulus or a boundary with a sharp visibility contrast.
The latter occurs if the air coming around one end of
the ridge flows past a polluted urban area such as in
the Lake Elsinore soaring area in southern California.
In very complex terrain, with ridges or ranges oriented
at different angles to one another, or with passes
between high peaks, small-scale convergence zones
can be found in adjacent valleys depending on wind
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Glider Flying Handbook(122)
